The present invention relates to a focus magnet for projection-type cathode ray tubes for use in high-definition video projectors, high-definition television sets, etc.
Conventionally, electrostatic-type electron beam converging apparatuses have been used for CRTs, and magnetic field-type convergence apparatuses are used only for special tubes such as X-ray tubes, magnetrons, travelling-wave tubes, etc. Recently, as a result of an increased demand for high-resolution CRTs such as high-definition television sets, etc., the magnetic field-type convergence apparatuses have been getting more widely used. The magnetic field-type convergence apparatuses include an electromagnet-type and a permanent magnet-type. The electromagnet-type convergence apparatuses are disadvantageous in that they are large in size and need power supplies. Accordingly, the permanent magnet-type convergence apparatuses are becoming a mainstay.
FIG. 4 shows one example of the permanent magnet-type convergence apparatuses, which comprises a hollow cylindrical permanent magnet 1, which is axially magnetized such that its end surfaces are provided with N and S magnetic poles, respectively. Fixed to both end surfaces of the permanent magnet 1 are yokes 2 each in the form of a hollow disc made of a ferromagnetic material. Disposed inside the permanent magnet 1 is a bobbin 4 which receives a coil 3. The coil 3 is connected to a lead wire (not shown). By this structure, when the coil 4 is energized, a magnetic field generated by the permanent magnet 1 can be adjusted such that the electron beams are converged on the center axis.
In the above conventional focus magnet, electron beams are converged on a center axis at a spot diameter of 0.3-0.35 mm. However, the spot diameter is required to be as small as 0.25-0.28 mm for recent high-definition projectors. If it is tried in the conventional focus magnet to focus electron beams to such a small spot diameter, halation appears due to a large spherical aberration. To obviate this problem, there was proposed a focus magnet comprising at least two ring-shaped permanent magnets with their opposite magnetic poles facing each other, a half-width of a magnetic flux density distribution along a z-axis (center axis) of the ring-shaped permanent magnets being 80-200% of an inner diameter of each permanent magnet (Japanese Patent Laid-Open No. 61-211940). In this focus magnet, a ratio of the half-width B.sub.w of the magnetic flux density distribution along a z-axis of the ring-shaped permanent magnets to the inner diameter of the permanent magnet, namely, H=B.sub.w /L is used as a parameter, and to increase "H" drastically, it is effective to use at least two ring-shaped permanent magnets with their opposite magnetic poles facing each other, and it is desirable to increase the "H" value to 0.8 or more to reduce the spherical aberration.
FIG. 3 is a vertical cross-sectional view of a main portion of the focus magnet according to the above proposal. In FIG. 3, the same parts as those in FIG. 4 are assigned with the same reference numerals as those in FIG. 4. In most cases, both of the two coils 3, 3 are used for dynamic control synchronized with vertical and horizontal scanning, and either one of the coils 3, 3 is used not only for dynamic control but also for static control. However, when the static control and the dynamic control are conducted by the same coil, it is necessary to use a control current consisting of a DC current superimposed thereover with an AC current. In this case, the variable range of the static control should be considerably wide by taking into consideration the unevenness of the magnetic force of the permanent magnet 1. For this purpose, it is preferable that the coils 3 have a large inductance. However, since a horizontal frequency is 15.75 kHz in the case of a television image, high voltage is required to carry out the focus modulation of the horizontal scanning.
To solve the above problem, there was proposed a focus magnet in which the static control and the dynamic control are carried out by separate coils (for instance, Japanese Utility Model Laid-Open No. 55-12576 and Japanese Patent Laid-Open No. 1-276546). In this focus magnet, a DC current is supplied to the static control coil, and an AC current is supplied to the dynamic control coil for the purpose of focus correction. Accordingly, the dynamic control coil may have a small inductance, leading to a low-voltage operation.
However, since both of the static control coil and the dynamic control coil are concentrically disposed inside the same permanent magnet, they are electromagnetically coupled to each other. Therefore, the AC current flowing through the dynamic control coil induces a current variation in the static control coil, which functions to offset the change of a magnetic flux for the dynamic control. As a result, the function of the dynamic control coil to change the focal length of electron beams is reduced. This means that a larger correction current should be supplied to the dynamic control coil. This is disadvantageous in that larger electric energy is consumed by the coils.